The present invention generally relates to voice and data communications systems and more specifically relates to a system wherein both voice and data signals are communicated between a central office terminal and various subscriber terminals.
Telephone voice transmission uses the frequency spectrum of approximately 300-3000 hertz. Supervisory and data signals are sometimes sent "out of band," almost always above the voice band--very seldom below the voice band.
Data over voice systems have several advantages, e.g. high speed data capability, very little interference with voice transmission, and the like. However, long subscriber loops require load coils to provide adequate voice band transmission and these transmission devices limit the high frequency performance of rural subscriber loops making data over voice impossible.
Data under voice transmission has not been widely used in telephone systems, even though it is not limited by load coils and would work very well on long rural loops. The problem with recent data under voice systems was the voice band noise that is caused by harmonic distortion (and other distortion products) of the low frequency signal. Conventional data under voice systems suffered from these noise characteristics such as "thumping" at the data rate, low frequency modulation of the natural system noise components, and multiple harmonic components.